The present invention relates generally to porous ceramic articles and methods for producing the same, and particularly to methods of producing porous ceramic articles using a compressible gas as the pore forming agent.
Porosity in ceramic substrates has been generated for decades by the use of organic pore formers. Typical organic pore formers such as graphite, starch, rice and cork have a large exotherm from 400-900° C. This exotherm results in cracking issues during calcination and very slow ramp rates. Organic pore formers typically produce a ceramic substrate with between 50% and 60% porosity. As the market pushes forward the demand for low cost and high porosity ceramic substrates continues. To achieve high porosity from conventional organic pore formers, a large amount of material must be added. Sometimes greater than 50% organic pore former was needed to achieve 65% porosity. In addition, for high porosity demands, this larger organic pore former loading does not produce equivalent resulting porosity.
Less conventional pore formers such as hydrogen peroxide have also been tried. The hydrogen peroxide is a liquid added in the wet batching process and is carried through the forming and drying steps where its concentration, interactions with other materials and the drying conditions cause its decomposition to form O2 or NO2 gas to form in the wet green body resulting in porosity.
Therefore it would be desirable to have a pore former that provided ceramic substrates having more consistent and greater porosity without the use of a particulate pore former. It would also be desirable if the pore former did not interact with other batch materials affecting rheology, batch flow and extrusion quality or generally cause difficulty in drying such as non-uniform dryness, longer drying schedules and/or cracking or complicate the firing process with exotherms requiring slower ramp rates and longer schedules, that could be controlled to give a desired pore size and would not require a “burn out” period. It would also be advantageous if such a pore former would produce a porous ceramic article having a greater porosity than that achieved with conventional particulate pore former.